Custom theming

[rlnorthcutt]

Problem statement

There's currently no supported way for a user to customize the UI's look beyond what ships by default.

What success looks like

A user can override the default look via a simple CSS file, and — longer term — manage full themes (create/import/export) as easily as changing a setting.

Impacted repositories

• omnideck

Acceptance criteria

• A blank custom.css file ships in the filesystem and is loaded by default, overriding core styles.
• A full theme system (CSS-based) lets users create, import, and export themes from settings.